1. Field of the Invention
The present invention relates to a sewing machine for automatically forming a double hem on free ends of garments that are circular or tubular in shape and for automatically stitching double folded portions of the garments.
2. Description of the Related Art
Japanese Patent No. 1640300 (Japanese Patent Publication No. 3-78) discloses a sewing machine for hemming double folded free ends of trousers, as shown in FIG. 14.
In this sewing machine 22a, prior to sewing, an inwardly folded generally tubular hem 60, indicated by a dotted line in FIG. 14, is turned over a lower feed roller 40 and a hem rotating roller 47a under tension, and is engaged at its inner peripheral surface with a folding guide 45. When the hem rotating roller 47a is rotated in the counterclockwise direction to move the hem 60 in a direction shown by an arrow Q, the hem 60 is double folded by the folding guide 45. Thereafter, as shown in FIG. 15, an upper feed roller 30 and a presser foot 28 are lowered to press the material of the hem 60 against a lower feed roller 40, and a sewing needle 27 is moved up and down by driving an upper or arm shaft (not shown). Accordingly, when the upper and lower feed rollers 30 and 40 are rotated in the directions shown by arrows N and S, respectively, that portion of the hem 60 which is sandwiched between the upper and lower feed rollers 30 and 40 is moved leftward, i.e., in the direction of the arrow Q, resulting in sewing of the double folded portion of the hem 60.
In this sewing machine 22a, however, when the double folded hem is formed prior to sewing, the folding guide 45 is positioned on the slack side of the hem rotating roller 47a that has moved to a tensioning position as shown by a solid line in FIG. 14. Because of this, it is likely that slackening occurs on the material of that portion of the hem 60 which is being moved from the hem rotating roller 47a toward the folding guide 45. As a result, the length of the hem folded by the folding guide 45 varies and, in some cases, does not fall within a desired range.
To overcome this problem, a freely rotatable hem guide roller 47 as shown in FIG. 8 is used, in place of the hem rotating roller 47a, together with upper and lower feed rollers 30 and 40 of a feed roller mechanism 57 shown in FIG. 5. As shown in FIG. 8, sandwiched between the upper and lower feed rollers 30 and 40 under pressure, the material of the hem 60 shown by a dotted line is moved in a direction shown by an arrow Q. In this case, the folding guide 45 is positioned on the light side with respect to the direction of rotation of the hem 60.
In the improved type above, the upper feed roller 30 is connected, via a flexible shaft 32, an upper feed shaft 33, a second gear 35, a first gear 34, and a one-way clutch 36, to a feed roller drive shaft 37 that rotates in synchronization with an arm shaft 25 for vertically moving a sewing needle 27, while the lower feed roller 40 is connected to the upper feed shaft 33 via a lower feed shaft 41, a fourth gear 43, and a third gear 42.
The third and fourth gears 42 and 43 are in mesh with each other so that the upper and lower feed rollers 30 and 40 may have the same circumferential speed. These rollers 30 and 40 are controlled by the first and second gears 34 and 35 so as to rotate at respective desired speeds. The rollers 30 and 40 are also rotated by a hem rotating motor 50 with a brake, independently of the rotation of the arm shaft 25, to first form a double fold of the hem prior to sewing.
The motor 50 has an output shaft 51 on which a sprocket 53 is mounted via a one-way clutch 52. The sprocket 53 is connected, via a chain 55, to another sprocket 54 fixedly mounted on the third gear 42. When the motor 50 rotates in a direction shown by an arrow in FIG. 6, the one-way clutch 52 acts to rotate the upper and lower feed rollers 30 and 40 via the chain 55 and the third gear 42.
More specifically, under the condition in which the inwardly single folded tubular hem 60 has been stretched under tension by the hem guide roller 47 from inside, as shown by a single-dotted chain line in FIG. 8, the folding guide 45 is moved towards an inner free end 61 of the hem 60 to roll it inwardly. The upper feed roller 30 is then lowered to press the hem 60 against the lower feed roller 40, and the motor 50 with the brake is activated to rotate the upper and lower feed rollers 30 and 40 in the directions of the arrows N and S, respectively, thereby moving the hem 60 in the direction of the arrow Q in FIG. 8 (the direction shown by an arrow in FIG. 9). As a result, a double fold 65 is first formed on the hem 60 and is subsequently transferred to a stitching area immediately below the sewing needle 27.
It is to be noted here that when the motor 50 with the brake is rotating, the first gear 34 runs idle with respect to the feed roller drive shaft 37 by the action of the one-way clutch 36.
The feed roller mechanism 57 referred to above, however, has the following drawbacks.
(a) After the hem 60 has been stretched by the hem guide roller 47 from inside, as shown in FIG. 8, and the folding guide 45 has been moved towards the inner free end 61 of the hem to roll it up, as shown in FIG. 10, the material of the hem 60 is sandwiched under pressure between and transferred by the upper and lower feed rollers 30 and 40, which are mutually connected via the third and fourth gears 42 and 43 at a fixed gear ratio, so that the double fold 65 may be formed on the hem 60, as shown in FIG. 13. Because the inner material 61 of the hem 60 is transferred under the condition in which it has received a resisting force greater than that received by the outer material 62 of the hem 60 by the folding action of the folding guide 45, the length of travel of the inner material 61 becomes shorter than that of the outer material 62, causing slackening on the inner material 61. Accordingly, when the sewing of the double folded portion of the hem 60 has been completed by rotating the hem 60 more than one revolution, the hem 60 is sometimes wrinkled or twisted. PA1 (b) Because the upper and lower feed rollers 30 and 40 are rotated via the first and second gears 34 and 35 by the feed roller drive shaft 37 that rotates together with the arm shaft 25, the gear ratio of the first and second gears 34 and 35 must be changed to change the pitch of seams. This requires a troublesome work to exchange the first and second gears 34 and 35.
To overcome this problem, the circumferential speeds of the upper and lower feed rollers 30 and 40 can be differentiated by changing the diameters thereof. In other words, if slackening occurs on the inner material 61, it is sufficient if the circumferential speed of the lower feed roller 40 is increased by enlarging the lower feed roller 40 in diameter relative to the upper feed roller 30. The degree of slackening on the inner material 61, however, differs according to the manner of weaving, flexibility, thickness, hardness and the like. To cope with differences in slackening, it is necessary to prepare a variety of lower feed rollers having different diameters, select an appropriate one of a diameter to remove slackening, and exchange them. This work is troublesome and time-consuming, and lowers the efficiency.
Furthermore, it is impossible to change the circumferential speed of the lower feed roller 40 during one round of the hem 60.